To increase the areal storage density of a magnetic recording device, the recording layer thereof may be provided with smaller and smaller individual magnetic grains. This reduction in grain size soon reaches a “superparamagnetic limit,” at which point the magnetic grains become thermally unstable and incapable of maintaining their magnetization. The thermal stability of the magnetic grains can be increased by increasing the magnetic anisotropy thereof (e.g., by utilizing materials with higher anisotropic constants). Increasing the magnetic anisotropy of the magnetic grains, however, increases their coercivity and therefore requires a stronger magnetic field to change the magnetic orientation of the grains (e.g., in a write operation).
Energy-assisted magnetic recording (EAMR) is used to address this challenge. In an EAMR system, a small spot where data is to be written is locally heated to reduce the coercivity of the magnetic grains therein for the duration of the write operation, thereby allowing materials with increased magnetic anisotropy to be used, and greater areal storage density to be exploited.
To reduce the size of the spot which is heated, a near field transducer (NFT) is used to concentrate optical energy in the near field to dimensions much smaller than the diffraction limit would otherwise allow. With a NFT, a nano-sized spot can be heated to assist the magnetic writing on the magnetic media. Due to the less-than-ideal NFT conversion efficiency, some of the optical energy provided on the NFT is absorbed by the materials near an air-bearing surface (ABS) surrounding the NFT, and heat-related protrusion and of the NFT and the surrounding materials may occur.